Start with the units behind the color
The US Environmental Protection Agency summarizes research finding urban daytime temperatures about 1–7°F higher than outlying areas and nighttime temperatures about 2–5°F higher. For temperature differences, multiply by 5/9: those ranges are approximately 0.6–3.9°C and 1.1–2.8°C. Do not subtract 32 when converting a difference; that offset applies to an absolute temperature.
These are broad research summaries, not predictions for a particular neighborhood or an instruction to add 3.9°C to every weather forecast. Reference sites, weather and measurement methods affect the observed contrast. A land-surface temperature layer also describes the radiating surface, rather than the air a person breathes.
A comparison can change without the city changing
Consider a teaching example: a shielded urban air sensor reads 32°C. A dry open reference site reads 30°C, giving an urban excess of 2°C. A shaded rural reference reads 28°C, giving an excess of 4°C. The urban measurement has not changed; the baseline has. These illustrative values are not observations from a named city.
This is why comparing two maps with different rural boundaries can produce a misleading claim that the heat island doubled. Keep sensor type, time, weather and reference definition consistent before interpreting changes in the contrast.
From a map to a practical cooling question
A hot paved schoolyard and a cooler tree-covered block suggest different surface energy balances: shade reduces incoming solar energy, and available water allows evaporative cooling. But a schoolyard satellite pixel does not reveal children’s exposure throughout the day. Shade at lunchtime, indoor temperature, activity duration and nighttime cooling require separate observations.
For deciding where to investigate cooling measures, compare repeated hot-day imagery with air measurements and the places people actually spend time. A one-day color ranking can help select survey sites; it cannot establish a health-effect ranking. Confirm whether a proposed tree or reflective-roof intervention addresses the measured variable rather than merely changing the map’s appearance.
US EPA, What Are Heat Islands? (updated 18 August 2026). Temperature ranges come from the EPA synthesis. Celsius conversions and the explicitly hypothetical baseline comparison are explained here; no local heat survey was conducted for this article. Source checked 30 October 2026; analysis updated on that date.
Define the heat-island comparison
An urban heat-island intensity is a difference between defined urban and reference observations at specified times. The result changes with reference site, weather, season, time of day and measurement type. “The city is 5°C hotter” is incomplete without those details.
What each record can support
| Record | Strength | Limitation |
|---|---|---|
| Satellite land-surface temperature | Spatial pattern on clear overpass dates | Not near-surface air temperature; clouds and emissivity matter |
| Fixed air-temperature station | Continuous local time series | Site exposure may not represent a neighborhood |
| Mobile traverse | Fine spatial comparison during a survey | Timing, route and sensor shielding can bias differences |
| Health record | Observed outcome in a population | Access, diagnosis and confounders affect interpretation |
Heat hazard is not the same as heat risk
Risk also depends on age, health, housing, work, electricity, cooling access, social isolation and the timing of exposure. A hot pixel does not identify who was present or whether indoor conditions were cooler or hotter.
Do not rank neighborhoods from one image
A single clear-sky daytime overpass can miss humid nights, indoor heat and short-lived weather. Compare multiple dates and pair surface maps with air sensors and population information.
Evidence task
For an urban heat map, record acquisition date and local time, sensor and product, pixel size, cloud screening, temperature units, urban/reference definition and whether the values are surface or air temperature.
Sources and limits
- US EPA: Heat Island Effect.
- USGS Landsat Surface Temperature: product and measurement context.
- US CDC: Heat and Health.
Sources accessed 10 October 2026. This page is not a current heat warning or medical guide.